This invention relates generally to expansible chamber devices which include a reciprocating piston, and more particularly to a piston having a non-metallic portion or portions.
Ceramic materials are available which are significantly harder, have a lower coefficient of friction, and have a lower coefficient of thermal expansion than the metallic materials conventionally used in reciprocating machinery. In some types of reciprocating machines, these materials can be used to advantage in the design of the piston/cylinder interface. Ceramic materials are particularly advantageous in cases where some component of the working fluid or fuel, which is present in the cylinder during operation, is incompatible with oil lubrication of the piston and piston rings.
The side walls of both the piston and cylinder can be made of ceramic material, so that there is only a ceramic-on-ceramic sliding interface. This arrangement takes advantage of the hardness and low coefficient of friction of the ceramic materials. It also provides a sliding interface which may have adequate life without lubrication, particularly if a crosshead is used to eliminate side loads from the crank mechanism.
A clearance is specified between the piston and cylinder to prevent seizing under the worst case of temperature difference between the piston and cylinder. If the piston and cylinder are made of ceramic material, the low coefficient of thermal expansion permits this clearance to be reduced. Then it is possible in some cases to eliminate the piston rings, and use the small clearance between the piston and cylinder to control blowby. This means that oil lubrication is not required for the piston or piston rings, and hence the lubricating oil can be eliminated from the cylinder spaced entirely, and isolated from the working fluid or fuel. In addition, the piston rings are the largest source of friction in most reciprocating machines. Hence elimination of the rings can improve brake efficiency directly.
A key problem with the use of ceramic materials in reciprocating machines is the design of an interface between the ceramic and metal parts which minimizes stresses under differential thermal expansion. Tensile stress in the ceramic material is particularly dangerous. Also fabrication of ceramic parts is expensive, particularly if much machining is involved. Therefore, it is desirable to keep the ceramic portion of the assembly small, and to keep the shape of the ceramic components as simple as possible. Only the sliding surface of the piston and cylinder can benefit from the properties of the ceramic materials.
The foregoing illustrates limitations known to exist in present devices. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.